Thermoelectric materials may be used to provide cooling and/or power generation according to the Peltier effect. Application of solid state thermoelectric cooling may be expected to improve the performance of electronics, photonics and sensors such as, for example, RF receiver front-ends, processors, programmable gate arrays, infrared (IR) imagers, laser diodes, light-emitting diodes, ultra-sensitive magnetic signature sensors, and/or superconducting electronics.
Traditional thermoelectric devices may be fabricated by assembling arrays of P and N thermoelectric elements (TE), as shown in perspective and cross-sectional views in FIGS. 27A and 27B, respectively. The TEs may be soldered to top and bottom high thermal conductivity substrates or headers. The headers may be oriented parallel to each other and may “sandwich” the TEs. Electrically conductive traces may be formed on the top and bottom headers and may form a series electrical circuit with alternating P and N elements. In this structure, the electrical current in the TEs and the flow of heat through the elements may both be substantially normal to the orientation of the headers, as shown in FIG. 27B.
The performance of a thermoelectric device may be a function of the figure(s)-of-merit (ZT) of the thermoelectric material(s) used in the device, with the figure-of-merit being given by:ZT=(α2Tσ/KT),  (equation 1)where α, T, σ, KT are the Seebeck coefficient, absolute temperature, electrical conductivity, and total thermal conductivity, respectively. The material-coefficient Z can be expressed in terms of lattice thermal conductivity (KL), electronic thermal conductivity (Ke) and carrier mobility (μ), for a given carrier density (d) and the corresponding α, yielding equation (2) below:Z=α2σ/(KL+Ke)=α2/[KL/(μdq)+L0T)],  (equation 2)where, L0 is the Lorenz number. State-of-the-art thermoelectric devices may use alloys, such as p-BixSb2-xTe3-ySey (x≈0.5, y≈0.12) and n-Bi2(SeyTe1-y)3 (y≈0.05) for the 200 degree K to 400 degree K temperature range. For certain alloys, KL may be reduced more strongly than μ leading to enhanced ZT. Also, DTmax (or ΔTmax) is another figure of merit for thermoelectric modules, and can be defined as the maximum operating temperature difference between the higher temperature side (“hot” side) and lower temperature side (“cold” side) of the thermoelectric film when no heat load is applied. An optimal current (Imax) is applied to achieve this maximum cooling. DTmax is defined with the hot (heat rejection) side held at a fixed temperature. When the heat load is increased from zero, the operating temperature difference, ΔT, is reduced. The ΔT is reduced to zero at a specific heat load, referred to as Qmax, which is quantified in watts. This is the maximum amount of heat that can be pumped by the thermoelectric device at a given temperature. Maximum heat pumping density is the Qmax per unit area of the device (Qmax/A).
In bulk thermoelectric materials, the c-axis of the crystal may have a high degree of tilt, such that, when thermoelectric modules are fabricated from the bulk materials, the heat/electrical conduction path may be substantially aligned with the direction of lowest resistivity in the bulk material. However, thinning of high tilt bulk material can be difficult, as the material is typically not durable enough to withstand the thinning process for thicknesses less than about 100 μm.
Controlled growth of thin-film thermoelectric materials has been developed. For example, V-VI based films, such as bismuth telluride (Bi2Te3) and/or antimony telluride (Sb2Te3)-based epitaxial films grown on gallium arsenide (GaAs) substrates, may be used in the fabrication of thin-film thermoelectrics. For thin film processes, the c-axis of the crystal is typically grown with a nearly 0 degree angle with respect to a perpendicular to the growth surface. As such, when thermoelectric modules are fabricated from thin-film thermoelectric materials, the heat/electrical conduction path may be substantially normal or perpendicular to the substrate growth surface, and substantially parallel to the growth direction. Some tilt in the c-axis of the thermoelectric film can be introduced by using an epitaxial growth process (such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE)) and providing an offcut substrate surface, as described, for example, in commonly-owned U.S. Pat. No. 7,804,019 to Pierce et al.